Studies are being performed to assess the role of nonspecific immune responses in the acquisition of age-dependent resistance to Sindbis virus encephalitis of mice. In vivo studies, on the reticuloendothelial cell system, natural killer cells, interferon, as well as the complement system are being evaluated in young, susceptible mice, and older resistant mice. In vitro studies are focused on the activation of the complement system (classical and alternative pathways) by Sinbis virus and the biological consequence of such activation.